Barium-and calcuim-based zeolitic adsorbent for gas purification in particular air

ABSTRACT

The invention concerns a calcium and barium cation-exchanged zeolitic adsorbent for use in a process for purifying and or separating a gas or gas mixture, in particular air, so as to eliminate impurities contained therein such as hydrocarbons and nitrogen oxides (NxOy), preferably, among ethylene, acetylene, butane and propane. Preferably, the adsorbent is an X or LSX zeolitic exchanged to between 10 and 90% with calcium cations and to between 10 and 90% with barium cations, the sum of barium and calcium cations present representing at least 20% of the exchangeable cations. The purification process is of the TSA type. Advantageously, CO 2  impurities and/or water vapour contained in the gas stream are eliminated as well. The invention also concerns a method for making such an adsorbent.

The present invention relates to a zeolite adsorbent exchanged withbarium and calcium cations, to a gas purification process using such anadsorbent, in particular an air pre-treatment prior to its separation bycryogenic distillation, and to its manufacturing process.

It is known that atmospheric air contains compounds that have to beremoved before the air is introduced into heat exchangers of the coldbox of an air separation unit, for example carbon dioxide (CO₂) and/orwater vapor (H₂O).

This is because, in the absence of such a pre-treatment of the air toremove the CO₂ and water vapor impurities therefrom, these impuritiescan condense and solidify as ice during cooling of the air to cryogenictemperature, hence resulting in problems of equipment blockage,especially in heat exchangers, distillation columns, etc.

Furthermore, it is also desirable to remove the hydrocarbon impuritieslikely to be present in the air so as to avoid any risk of damaging theequipment.

It is preferable also to remove the nitrogen oxides likely to be foundin the air, such as N₂O, so as to prevent them from being concentratedand deposited in the reboilers of the cryogenic distillation plants,with the risk of blocking them.

Currently, this air pre-treatment is carried out by adsorption using,depending on the case, a TSA (Temperature Swing Adsorption) process or aPSA (Pressure Swing Adsorption) process.

TSA air purification processes have been described for example indocuments U.S. Pat. No. 3,738,084 and FR-A-7 725 845.

In general, the CO₂ and water vapor (H₂O) impurities are removed overone or more beds of adsorbents, preferably several beds of adsorbents,namely in general a first adsorbent intended to preferentially stopwater, for example a bed of activated alumina, of silica gel orzeolites, and a second bed of adsorbent for preferentially stopping CO₂,for example a zeolite. This is because effective removal of CO₂ andwater vapor contained in the air over one and the same bed of adsorbentis not easily accomplished, as water has a markedly greater affinity forthe adsorbents than CO₂ has, and it is therefore standard practice touse at least two beds or layers of adsorbents of different type.

In this regard, mention may, for example, be made of the documents U.S.Pat. No. 5,531,808, U.S. Pat. No. 5,587,003 and U.S. Pat. No. 4,233,038.

In the document “zeolite molecular sieves”, Krieger Publishing Company,1984, page 612, D. W. Breck recommends the use of an unexchanged13X-type zeolite (sodium form) to remove small amounts of CO₂ andpossibly of water as it has a strong affinity and selectivity for thesepolar molecules.

However, the 13X zeolite does not make it possible to stop, in a mannerequal to or better than CO₂, all the harmful molecules likely to bepresent in a gas stream, in particular hydrocarbons and nitrogen oxides,as recalled by the following: E. Alpay, “Adsorption parameters forstrongly adsorbed hydrocarbon vapours on some commercial adsorbents”,Gas Sep. & Purif., Vol. 10, No. 1, pp. 25 (1996); G. Calleja,“Multicomponent adsorption equilibrium of ethylene, propane, propyleneand CO₂ on 13X zeolite”, Gas Sep. & Purif., Vol. 8, No. 4, p. 247(1994); V. R. Choudhary, “Sorption isotherms of methane, ethane, etheneand carbon dioxide on NaX, NaY and Na-mordenite Zeolites”, J. Chem. Soc.Faraday Trans., 91(17), p. 2935 (1995); and A. Cointot, P. Cartaud andC. Clavaud, “Etude de l'adsorption du protoxyde d'azote par différentstamis moléculaires”, [Study of nitrous oxide adsorption by variousmolecular sieves]”, Journal de Chimie Physique, Vol. 71, No. 5, p.765-770 (1974).

It therefore follows that an up-stream industrial air purification unitstrictly designed for stopping carbon dioxide using a standard zeolite,typically a 13X or 5A zeolite, stops only partly, or even not at all,ethylene, propane, other hydrocarbons and nitrous oxide, as recalled inDr J. Reyhing's document “Removing hydrocarbons from the process air ofair-separation plants using molecular-sieve adsorbers”, Linde Reports onScience and Technology, 36/1983.

As regards stopping nitrous oxide, the ineffectiveness of the 5A zeolitefor stopping N₂O compared with CO₂ was demonstrated by U. Wenning in“Nitrous oxide in air separation plants”, MUST'96, Munich Meeting on AirSeparation Technology, 10-11 October 1996.

One solution was proposed in document EP-A-1 064 978, which discloses anadsorbent consisting of an X or LSX (low-silica X) zeolite exchanged toat least 30%, preferably at least 75%, with barium cations, whichzeolite can be used to remove certain impurities from the air, inparticular nitrous oxide, propane and ethylene, the residual cationsbeing sodium and/or potassium cations.

The zeolites disclosed therein are obtained by an ion exchange processwhich is quite complex as soon as the degree of exchange with barium hasto exceed 50%.

This is because a zeolite usually consists of a negatively chargedaluminosilicate framework in which compensating cations occupy positionsdefined by the charge of the cation, by its size and polarizability, andby the charge of the zeolitic framework and its crystalline structure.

According to that document, the exchanged zeolite is obtained by ionexchange starting with an X or LSX zeolite initially containing sodium(Na⁺), to end up with a zeolite containing at least 30% barium.

However, Ba²⁺ cations are voluminous and cannot reach certaincrystallographic sites occupied by the Na⁺ cations, this having theeffect of limiting the degree of exchange to about 75% at most.

To reach higher values (>75%), it is necessary to perform additionaloperations intended to force the cations to migrate towards the barelyaccessible sites. The applicable procedure consists in carrying out afirst exchange with barium and then in drying the zeolite and heating itto at least 200° C. The Ba²⁺ cations are then stripped of their train ofsolvation water molecules and, moreover, they are subjected to greaterthermal agitation. Migration towards the inaccessible sites can thentake place.

It should be noted that these sites are thermodynamically favored andthat only steric hindrance prevents the cations from occupying them.

Moreover, it seems that it is the accessible sites, i.e. the II and II′sites, which give the barium cations their remarkable properties.

The adsorbent disclosed in EP-A-1 064 978 cannot therefore be regardedas completely satisfactory from the technical standpoint—the manyhydrothermal treatments to which it has to be subjected damage itsstructure—and it is also very expensive compared with the adsorbentscurrently used because of the high amount of barium that has to be usedto perform the ion exchange.

From there, the problem that then arises is to be able to have azeolite-type adsorbent which is approximately as effective for removinghydrocarbons and, if possible, more effective for removing nitrogenoxides in a gas stream to be purified, in particular air, but which iseasier to manufacture and therefore of lower cost than that known fromEP-A-1 064 978.

The object of the present invention is therefore to try to solve thisproblem by providing an improved zeolite adsorbent that can be used topurify gases, such as air, and its manufacturing process.

The solution provided by the invention is therefore a zeolite adsorbentexchanged with calcium cations and with barium cations.

Within the context of the invention, the expression “exchanged withcations” is understood to mean that the cations in question are thoseassociated with AlO₂ ⁻ tetrahedral units of the zeolite (zeoliticphase), which exchanged cations play a role in the mechanism ofadsorbing the gaseous compounds to be removed.

Likewise, the term “exchangeable cations” is understood to mean cationsthat can be substituted or replaced with other cations by performing anion exchange process.

The expression “degree of exchange of a cation X” is understood to meanthe number of charges carried by the cations X present in the zeolitewith respect to the total number of charges of all of the cations. Thedegree of exchange varies between 0 and 100%. The total positive chargecarried by the cations is equal to the total negative charge carried bythe AlO₂ ⁻ groups. The stoichiometric amount corresponds to this totalcharge.

Depending on the case, the adsorbent of the invention may include one ormore of the following technical features:

-   -   it contains or is formed from X or LSX (low-silica X) zeolite;    -   it is exchanged by 10 to 90% with calcium cations and by 10 to        90% with barium cations, the sum of the barium and calcium        cations present representing at least 20% of the exchangeable        cations;    -   it is exchanged by 20 to 70% with calcium cations, preferably by        30 to 50% by calcium cations;    -   it is exchanged by 20 to 70% with barium cations, preferably by        30 to 50% with barium cations;    -   the sum of the barium and calcium cations present represents at        least 30% of the exchangeable cations, preferably at least 40%        of the exchangeable cations;    -   it also contains residual sodium and/or potassium cations,        preferably the sum of the residual sodium and/or potassium        cations representing less than 40% of the exchangeable cations,        preferably less than 30% of the exchangeable cations and even        more preferably less than 20% of the exchangeable cations;    -   it comprises a zeolitic phase and at least one binder        representing less than 30% by weight of the total weight of the        adsorbent. Preferably, the zeolitic phase represents at least        70% by weight of the total adsorption mass;    -   it contains 15 to 65% calcium, 25 to 80% barium and residual        sodium and/or potassium cations, the sum of the barium, calcium,        sodium and potassium cations present representing at least 80%        of the cations present in the adsorbent, preferably the sum of        the barium, calcium, sodium and potassium cations present        representing from 90% to 100% of the cations present in the        adsorbent;    -   the zeolite has a pore size of between 4 and 10 Å, preferably        between 5 and 8 Å;    -   the zeolite furthermore contains at least one cation of group IA        or IIA; and    -   the zeolite has an Si/Al ratio of between 1 and 1.50, preferably        between 1 and 1.26.

The adsorbent of the invention can be used in a process for purifying orseparating a gas or gas mixture, particularly air.

Depending on the case, the gas purification process of the invention mayinclude one or more of the following technical features:

-   -   at least one impurity chosen from hydrocarbons and nitrogen        oxides (N_(x)O_(y)), preferably from ethylene, acetylene, butane        and propane, is chosen;    -   the CO₂ and/or water vapor are/is also removed over the zeolite        of the invention or over a bed or several other beds, stopping        these harmful gaseous compounds. For example, a bed of activated        alumina may be used to adsorb water, this bed being placed        upstream of the bed of Ca/Ba zeolite according to the invention        when the direction of flow of the gas stream to be purified is        taken into consideration;    -   the process is chosen from PSA or TSA, preferably TSA,        processes;    -   the stream of air stripped of at least some of the said        impurities is subjected to at least one cryogenic distillation        step so as to produce nitrogen, oxygen and/or argon;    -   the gas stream is at a temperature of between −40° C. and +80°        C., preferably between −10° C. and +50° C.;    -   the adsorption pressure is between 2 bar and 30 bar, preferably        between 4 bar and 20 bar;    -   the desorption pressure is between 0.5 bar and 10 bar,        preferably between 1 bar and 6 bar;    -   the flow rate of the gas stream is between 1 and 10⁶ Sm³/h,        preferably between 10⁴ and 5×10⁵ Sm³/h;    -   the regeneration temperature is between 60° C. and 400° C.,        preferably between 80° C. and 300° C.;    -   the regeneration gas for the adsorbent is nitrogen or a        nitrogen/oxygen mixture containing a small proportion of oxygen        (a few vol %), preferably the nitrogen/oxygen mixture used to        regenerate the adsorbent being a waste gas coming from a        cryogenic air separation unit;    -   the process of the invention is carried out in at least one        adsorber, preferably in at least two adsorbers operating        alternately; and    -   the TSA process of the invention operates by a purification        cycle, each cycle comprising the following successive steps:    -   1) purification of the air by adsorption of the impurities at        superatmospheric pressure and at ambient temperature over the        adsorbent,    -   2) depressurization, preferably countercurrent depressurization,        of the adsorber down to atmospheric pressure or to below        atmospheric pressure,    -   3) regeneration, preferably countercurrent regeneration, of the        adsorbent at atmospheric pressure, especially by waste gases,        typically impure nitrogen coming from an air separation unit and        heated up to a temperature of greater than +80° C. by means of        one or more heat exchangers,    -   4) cooling the adsorbent to ambient, superambient or subambient        temperature, especially by continuing to introduce thereinto,        preferably countercurrently, the said waste gas coming from the        air separation unit, but not heated and

5) repressurization, preferably countercurrent repressurization, of theadsorber with purified air coming, for example, from another adsorber,which is in the production phase.

The invention also relates to a process for manufacturing a zeoliteadsorbent exchange with calcium and barium cations, in which:

-   -   (a) a zeolite containing sodium and/or potassium cations is        subjected to a first ion exchange by bringing the said zeolite        into contact with a solution containing calcium cations;    -   (b) the zeolite from step (a) is subjected to a second ion        exchange by bringing the said zeolite into contact with a        solution containing barium cations;    -   (c) if necessary, steps (a) and/or (b) are repeated until the        desired degree of exchange for each of the said barium and        calcium cations has been reached; and    -   (d) a zeolite exchanged with calcium and barium cations is        recovered.

Depending on the case, the manufacturing process of the invention mayinclude one or more of the following technical features:

-   -   after each step (a) and after each step (b), the exchange        solution is drained and, optionally, the zeolite obtained is        rinsed;    -   after step (a) and/or after step (b), a heat treatment is        carried out on the zeolite by heating to more than ₉₅° C. for a        time long enough to ensure that the cations migrate towards the        AlO₂ ⁻sites of the zeolite, the heating preferably being        maintained for 15 minutes to 12 hours;    -   after each heat treatment and before any subsequent new ion        exchange step, the zeolite is rehydrated, preferably with        demineralized water;    -   the zeolite is in powder form or in agglomerated form;    -   the saline solutions used in one step (a) and/or in step (b) are        recovered and reused during another subsequent step (a) and/or        (b), in particular the barium salt solution;    -   the zeolite is in powder form or in agglomerated form;    -   the saline solutions used in step (a) and/or step (b) are        recovered and reused during step (c), in particular the barium        salt solution;    -   an amount of barium ranging from 110% to 200% of the amount        introduced into the zeolite is consumed;    -   steps (a) and (b) are successive, in any order, or, depending on        the case, carried out at the same time and in a single step        using a saline solution formed from a mixture of calcium and        barium salts;    -   steps (a) and (b) are carried out in an ion exchange column or        in a stirred (batch) reactor;    -   steps (a) and (b) are carried out simultaneously and in a single        step using a solution containing calcium and barium cations;    -   the starting zeolite is an X or LSX zeolite in sodium/potassium        form (i.e. not exchanged) which optionally undergoes prior to        step (a) a preliminary step in which it is treated with an        ammonium and/or sodium solution;    -   after step (d), the zeolite exchanged with calcium and barium        cations undergoes an activation step in a water-free medium, by        purging with a dry gas or vacuum, at a temperature greater than        or equal to 200° C.; and    -   the zeolite is mixed with a binder, such as clay, silica gel or        the like, in order to obtain agglomerated zeolite particles and,        optionally, the binder is converted into a zeolite in order to        form particles with no binder. This operation takes place before        or after, preferably before, the Ca/Ba ion exchange.

The invention will now become more clearly understood thanks to theexplanations and comparative examples given below as illustration andwith reference to the appended figures.

The inventors of the present invention have demonstrated that, byperforming a calcium exchange and then a barium exchange on an X or LSXstarting zeolite, or the other way round, what is obtained at the end isan adsorbent exchanged with barium and calcium cations which, on the onehand, is easier and less expensive to manufacture than if it were solelyexchanged with barium and, on the other hand, which could be usedeffectively to separate or purify gas mixtures, particularly air, byselective adsorption of the impurities contained in this mixture,particularly hydrocarbons and nitrogen oxides.

The process for manufacturing such a zeolite exchanged with calcium andbarium consists in performing, on a zeolite, preferably an X or LSXzeolite, in sodium and/or potassium form, an ion exchange with calciumcations, to an extent sufficient to replace the sodium cations whichoccupy the barely accessible sites. This is because it appears that theCa²⁺ cations occupy all the sites without any difficulty, unlike thebarium cations, and that the barely accessible sites are also mostfavorable from the thermodynamic standpoint; an amount of calcium justsufficient to occupy the barely accessible sites is thereforeintroduced, i.e. typically 30 to 50% of the available sites in thezeolite (AlO₂ ⁻ units).

After exchange with calcium, an exchange with barium is carried out, theBa²⁺ cations occupying the accessible sites.

An alternative way of carrying out the process consists in performingthe exchange in a signal go, using a mixed calcium/barium solution.

The product thus obtained has, after activation, properties similar tothose of the zeolite exchanged with pure barium, with also an additionalcapacity for nitrogen oxides.

Moreover, the manufacture of this novel product turns out to be mucheasier and more favorable to the preservation of the zeolite structure,successive ion exchanges and activations always having the effect ofslightly degrading the zeolite by hydrothermal attack.

In addition, the cost of the final product is appreciably lowered by twoeffects, namely the much simpler manufacturing process and the lowercost of calcium compared with barium.

The ion exchange is carried out on the starting zeolite, which may be anX or LSX zeolite, initially containing sodium and/or potassium which areeasily exchangeable cations, the zeolite possibly being innon-agglomerated powder form or else formed into extrudates, into beadsor any other form.

To carry out the ion exchanges, a solution of calcium and/or bariumsalts, such as a chloride solution, with a pH of less than about 6 ispreferably used.

Contact between the zeolite and the saline solution takes place, forexample, by immersing all of the zeolite for as short a time aspossible, so as to ensure homogeneous ion exchange in the zeolite.

Alternatively, the zeolite powder is placed into a stirred suspension inwater and then the solution of calcium and/or barium salts is slowlyadded, with stirring which is sufficient to distribute the solutionthroughout the entire volume in suspension.

In all cases, the contact must be carried out under conditions in whichthe calcium and/or barium salt is distributed throughout the entirevolume of zeolite, before the exchange has had time to take place,thereby ensuring that the calcium and/or barium is distributedhomogeneously throughout the mass of the zeolite.

The salt molarities are between 1M and 0.01M, the temperature is between20° C. and 100° C. and the contact time is between 20 minutes and 3hours.

After exchange, the zeolite is rinsed with pure water, drained and thenactivated between about 300° C. and 450° C. in a stream of dry gas orvacuum, under conditions which minimize contact between the steamreleased and the zeolite.

EXAMPLES WITH ADSORBENTS ACCORDING TO THE INVENTION

To demonstrate the effectiveness of a purification process according tothe invention, a break-through curve (FIG. 1) was produced for zeoliteparticles of a CaBaX adsorbent according to the invention.

To do this, a stream of nitrogen contaminated by the impuritiesmentioned below was introduced into the inlet of a bed of adsorbentcontaining the said CaBaX particles and the concentration of theseimpurities at the outlet (downstream) of the said bed was continuouslymeasured over time.

The operating conditions for the tests were the following:

-   -   stream of nitrogen contaminated by 400 ppm by volume of CO₂        (curve C1), 1.3 ppmv of N₂O and 1 ppmv of C₂H₄, C₃H₈, C₂H₂ and        C₃H₆;    -   adsorption pressure: 6 bar (6×10⁵ Pa);    -   gas stream at a temperature of around 20° C.;    -   flow rate of the gas stream: 10 Sm³/h; and

400 g of particles of a CaBaX zeolite exchanged by about 40% with Ca andby 40% with Ba and containing Na and K cations for the balance (i.e.making up to 100% of the exchangeable cations).

The results obtained are set out in FIG. 1 which shows that all thesecondary impurities (C₂H₄, C₃H₈, C₂H₂ and C₃H₆) break through after theCO₂ (curve C2).

Moreover, it should be noted that acetylene (curve x) and propylene(curve o) do not break through after more than 250 minutes.

Furthermore, the figure shows that the CO₂ and N₂O impurities areremoved almost simultaneously for more than 60 minutes (curves C2 andC3, respectively), that is to say without any regeneration over thisentire time period.

More specifically, FIG. 1 also gives (at the bottom right) an enlargedview of curves C2 and C3, showing that in fact N₂O breaks through afterCO₂.

Other similar tests were carried out using CaBaX zeolites according tothe invention, but with different cation contents, in particular:

-   -   a CaBaX zeolite exchanged by 35% with calcium and by 50% with        barium and containing Na and K cations for the balance (i.e.        about 15%); and    -   a CaBaX zeolite exchanged by 45% with calcium and by 40% with        barium and containing Na and K cations for the balance (i.e.        about 15%).

During these tests, the operating conditions were the same aspreviously.

The results obtained are very similar to those given in FIG. 1, inparticular, here again, it may be seen that the secondary impuritiesbreak through after the CO₂.

COMPARATIVE EXAMPLES WITH ADSORBENTS ACCORDING TO THE PRIOR ART

For comparison, several tests with adsorbents according to the prior artwere carried out under the same conditions as those for the CaBaXzeolites of the example according to the invention.

More specifically, these comparative tests were carried out:

-   -   with an unexchanged 13X zeolite, i.e. containing only Na and K        cations;    -   with a CaX zeolite exchanged by 60% with calcium and containing        Na and K cations for the balance; and    -   with a BaX zeolite exchanged by 94% with barium and containing        Na and K cations for the balance.

The results obtained with these adsorbents are set out in FIGS. 2 to 4and show that:

-   -   with the unexchanged 13X zeolite (FIG. 2), ethylene, propane and        N₂O break through well before CO₂. It should be noted that, in        this case, the stream of gas tested did not contain C₂H₂ and        C₃H₆ compounds;    -   with the CaX zeolite exchanged by 60% (FIG. 4), although        ethylene is quite well stopped it seems that N₂O also breaks        through after CO₂ and that propane is not, however, removed; and    -   with the BaX zeolite exchanged by 94% (FIG. 3), ethylene and N₂O        are much less well stopped than with the CaBaX adsorbents        according to the invention that were tested. It should also be        noted that exchange by 94% with barium is complicated and        expensive to obtain.

It is apparent from the above examples that the zeolites of the presentinvention exchanged with calcium and barium cations are particularlyeffective when used in a TSA process to purify atmospheric air of itsCO₂ and N₂O, but also C₂H₄, C₃H₈, C₂H₂ and C₃H₆, impurities.

1-21 (canceled).
 22. A zeolite adsorbent comprising calcium and bariumcations.
 23. The adsorbent of claim 22, wherein said adsorbent comprisesX or LSX zeolite or is produced from X or LSX zeolite.
 24. The adsorbentof claim 22, wherein said adsorbent comprises exchangeable cations inthe amount of from 10 to 90% calcium cations, from 10 to 90% with bariumcations, and wherein the sum of said barium and said calcium cationsrepresent at least 20% of the total exchangeable cations available insaid adsorbent.
 25. The adsorbent of claim 24, wherein said adsorbent isexchanged with 20 to 70% with said calcium cations.
 26. The adsorbent ofclaim 24, wherein said adsorbent is exchanged with 20 to 70% with saidbarium cations.
 27. The adsorbent of claim 24, wherein the sum of saidbarium and said calcium cations present on said adsorbent represents atleast 30% of the exchangeable cations.
 28. The adsorbent of claim 24,further comprising residual sodium and/or potassium cations, wherein thesum of the residual sodium and/or potassium cations represents less than40% of the exchangeable cations.
 29. The adsorbent of claim 22, furthercomprising at least one binder, wherein said binder represents less than30% by weight of the total weight of said adsorbent.
 30. The adsorbentof claim 28, further comprising 15 to 65% said calcium, 25 to 80% saidbarium, and said residual sodium and/or potassium cations, wherein thesum of said barium, calcium, sodium, and potassium cations presentrepresents at least 80% of the cations present in said adsorbent. 31.The adsorbent of claim 30, wherein the sum of said barium, calcium,sodium, and potassium cations present represents from 90% to 100% of thecations present in the adsorbent.
 32. A process for purifying orseparating a gas or gas mixture comprising treating said gas or gasmixture with a zeolite adsorbent comprising calcium and barium cations.33. The process of claim 32, wherein said gas is air.
 34. The process ofclaim 32, wherein at least one impurity removed from said gaseousmixture is selected from the group consisting of hydrocarbons andnitrogen oxides (N_(x)O_(y)).
 35. The process of claim 34, wherein saidhydrocarbon is selected from the group consisting of ethylene,acetylene, butane, and propane.
 36. The process of claim 32, wherein atleast one of CO₂ or water vapor is also removed.
 37. The process ofclaim 32, wherein the gaseous mixture is treated in a process selectedfrom the group consisting of a PSA or TSA process.
 38. The process ofclaim 32, wherein at least part of said gas or gas mixture furthercomprises subjecting said gas or gas mixture to at least one cryogenicdistillation step.
 39. The process for manufacturing a zeolite adsorbentcomprising calcium and barium cations, comprising: (a) subjecting azeolite containing sodium and/or potassium cations to a first ionexchange by bringing said zeolite into contact with a solutioncontaining calcium cations; (b) subjecting said zeolite from step (a) toa second ion exchange by bringing said zeolite into contact with asolution containing barium cations; (c) repeating as necessary steps (a)and/or (b) until the desired degree of exchange for each of the saidbarium and calcium cations has been reached; and (d) recovering azeolite exchanged with calcium and barium cations.
 40. The process ofclaim 39, further comprising steps (a) and (b) being carried outsimultaneously and in a single step using a solution containing calciumand barium cations and the zeolite is of the X or LSX type.
 41. Theprocess of claim 39, further comprising heating the zeolite prior tostep (a) to more than 95° C. for a time long enough to ensure that thecations migrate towards the AlO₂ ⁻sites of the zeolite.
 42. The processof claim 41, further comprising rinsing said zeolite after each heattreatment step and before any subsequent new ion exchange step.